The invention relates to an illumination unit comprising an electrodeless low-pressure discharge lamp and a high-frequency electric supply device, which lamp is provided with a discharge vessel sealed in a gastight manner and containing an ionizable filling and with a coil comprising turns of a primary winding and of a secondary winding, which supply device is provided with input terminals, with an output terminal which is electrically neutral with respect to ground and which is connected to a first end of the primary winding, and with a further output terminal which is connected to a second end of the primary winding, a high-frequency magnetic field being generated by the primary winding in a nominal operating condition, which field maintains an electric discharge in the discharge vessel and induces a potential gradient in the secondary winding in a direction from a first end which is electrically neutral with respect to ground to a second, free end of the secondary winding, which gradient is oppositely directed to the potential gradient from the first to the second end in the primary winding.
The invention also relates to an electrodeless low-pressure discharge lamp suitable for use in the illumination unit.
The invention further relates to a coil suitable for use in the illumination unit.
Such an illumination unit is known from EP 0.162.504 A1. The discharge vessel of the lamp of this illumination unit has a luminescent layer at the inside and is provided with a filling comprising mercury. The coil, which has 13 turns in a primary winding over a length of 25 mm and 14.5 turns in a secondary winding over a length of 30 mm, is provided around a core of soft magnetic material of 50 mm length.
A potential distribution across the coil resulting from the potential gradient occurring in each of the windings causes an electric field which is of importance for lamp ignition.
The asymmetrical supply device used in the illumination unit, where one of the terminals has a potential which at least substantially corresponds to ground and the other has a potential different therefrom, may be comparatively simple compared with a symmetrical supply device, i.e. a supply device with connection terminals which have mutually opposite potentials with respect to ground.
Although an asymmetrical supply device is used in the known illumination unit, the presence of the secondary winding results in a potential distribution across the coil which is at least substantially balanced relative to ground, so that the average potential over the coil surface is approximately equal to that of ground. Interference effects in the mains and in the environment have been limited to an acceptable level thereby.
A disadvantage of the known illumination unit is that the lamp ignites with comparatively great difficulty compared with a lamp of an illumination unit in which the coil has no secondary winding. This has the result that the ignition circuit is heavily loaded during a prolonged period, which may lead to a reduced operating life. It is true that the ignition time can be shortened by offering a higher ignition voltage, but this requires the use of comparatively expensive components.